Combine butter and chocolate in a deep half hotel pan with the butter underneath the chocolate. Microwave for one minute at a time, stirring well between, until mixture is completely melted and smooth. Add cornstarch, sugar, and salt and stir aggressively, ensuring that no lumps of cornstarch or sugar remain. Add eggs and egg yolks and stir until mixture is uniform.

Prepare truffle ring molds by wrapping aluminum foil tightly around the bottoms and placing a ring of parchment paper inside. Spray well. Pipe truffle batter into molds to just below the top of the rings.

Bake at 325 for 40 minutes, turning very carefully halfway through. Do not bake anything else off at the same time, unless it will be baking for the entire 40 minutes. If the oven is opened during baking for more than a few seconds or the door is closed too hard, the truffle cakes will fall and not have a uniform flat top. Be very careful during baking.

Remove from ring molds prior to refrigerating. If cakes are still hot when removed, leave parchment rings around them for stability.